1. Field of the Invention
The present invention relates to an optical fiber suitable as an optical transmission path and a method for making such an optical fiber.
2. Related Background Art
Conventionally, a dispersion managed fiber as an optical transmission path composed of plural fiber sections having different fiber characteristics at respective sections and can solve problems not solvable by an optical fiber composed of a single kind of section is disclosed in U.S. Pat. No. 5,894,537, for example. In this dispersion managed fiber, a dispersion managed transmission path is constituted of sections having a positive chromatic dispersion and sections having a negative chromatic dispersion, so that the deterioration in transmission characteristics due to the nonlinear optical interaction among optical signals having different wavelengths and the distortion of optical pulses due to total chromatic dispersion can be reduced simultaneously.
Among methods for making such a dispersion managed fiber, following two methods are provided, for example. The first is a method which changes the refractive index of the core region along the fiber axis. For example, the core region is doped with such materials that the refractive index of core region changes by exposure to ultraviolet radiation. The fiber is exposed to ultraviolet radiation after fiber drawing so as to obtain a desired refractive index. The second is a method which changes the diameter of the core region along the fiber axis.
However, both of the above-mentioned two methods have problems as follows. In the first method, usually, since the change in refractive index induced by exposure to ultraviolet radiation is approximately 10−3 and hence is small, it is difficult to change the chromatic dispersion widely along the fiber axis. Accordingly, the absolute value of the local chromatic dispersion cannot be increased and hence, it is impossible to sufficiently suppress the nonlinear optical interaction among optical signals having different wavelengths. Further, it is also difficult to change the sign of the chromatic dispersion slope along the fiber axis so that the total chromatic dispersion slope increases. As a result, the wavelength band with sufficiently small total chromatic dispersion gets narrow and hence, the capacity of the transmission path becomes small.
Further, in the second method, it is difficult to have the cross-sectional distribution of refractive index change drastically along the fiber axis. To realize a negative chromatic dispersion slope, the refractive index distribution having a depressed portion, i. e., a refractive index distribution having, between the core region having a high refractive index and the outer cladding region having a low refractive index, an inner cladding region (the depressed portion) whose refractive index is lower than the outer cladding region is suitable. On the other hand, to realize a positive chromatic dispersion slope, the refractive index distribution having no depressed portion, i. e., the refractive index distribution where the refractive index takes the minimum in the outer cladding region is suitable. However, it is usually difficult to make a preform where a section has a refractive index distribution having a depressed portion and another section has a refractive index distribution having no depressed portion. Accordingly, the absolute value of total chromatic dispersion slope becomes large and the wavelength band with sufficiently small absolute value of total chromatic dispersion becomes narrow.
Further, as the change in the chromatic dispersion along the fiber axis becomes steeper, the fabrication of the fiber becomes more difficult. For example, when the preform diameter is 50 mm and the fiber diameter is 125 μm, to realize a change in chromatic dispersion at a period of 640 m along the fiber axis, it is necessary to change the ratio of the core diameter to the cladding diameter in the preform at a period of 4 mm along the preform axis. Accordingly, in case of grinding the core preform, a minute processing technique becomes necessary, and in case of elongating the core preform, a highly position-selective heating technique becomes necessary. Further, the shorter the period of the change in the chromatic dispersion along the fiber axis, the number of the parts in the preform to be processed increases so that the fabrication becomes laborious.
Conventionally, there has been known a dispersion compensating fiber which has negative chromatic dispersion and negative chromatic dispersion slope to compensate for positive chromatic dispersion and positive chromatic dispersion slope as disclosed in U.S. Pat. No. 5,995,695. However, a dispersion compensating fiber having positive chromatic dispersion and negative chromatic dispersion slope has not been known and hence, it has been difficult to compensate for negative chromatic dispersion and positive chromatic dispersion slope. A dispersion managed fiber including sections having positive chromatic dispersion and negative chromatic dispersion slope and sections having negative chromatic dispersion and positive chromatic dispersion slope has not been known either. Accordingly, in the conventional dispersion managed fiber, locally-zero-dispersion wavelength, at which local chromatic dispersion becomes substantially zero, is present in the short wavelength side of the operating wavelength range. The band in the vicinity of this wavelength is not suitable for the wavelength division multiplexing transmission because of the deterioration of the transmission quality due to the four-wave mixing or the cross phase modulation and hence, the conventional dispersion managed fiber cannot expand its operating wavelength range to the short-wavelength side.
So-called microstructured optical fiber, which has a high degree of freedom in setting the local chromatic dispersion is disclosed in U.S. Pat. No. 5,802,236. This microstructured optical fiber has microstructures (usually voids) in a cladding region and it is possible to increase the effective refractive index difference between the core region and the cladding region. As a result, this optical fiber can realize large absolute value of the chromatic dispersion and small mode field diameter.
A method for manufacturing such a microstructured optical fiber is disclosed in U.S. Pat. No. 5,802,236, wherein tubes and a rod are bundled to form a preform from which a microstructured fiber is drawn. Another method of making a microstructured fiber is disclosed in the International Publication WO00/16141 wherein a plurality of rods of given shape are bundled to form a preform from which a microstructured fiber is drawn.